Hello all: New Tableau user here (still in my trial period) attempting to learn the "lay of the land".  I am very excited about the potential with Tableau and my head is spinning with ways that it can help my business.  I've hit a roadblock which I'm hoping that someone can help me with.  I've queried an ODBC database containing some accounting figures and have started to put together a profit and loss statement.  Right now, I have all of my income accounts and expense accounts showing accurate totals by month for 2012.  While I can show subtotals without issue, I am not sure how to show a basic net income number (total income- total expense). I also need to perform calculations on that net income number with some of the other data shown already aggregated the worksheet.  For example, I would take net income total and then add Depreciation to it, and then show that total on a separate row.  I'm looking for what basically amounts to a calculated item (not field) in Excel PivotTables.  The screenshot attached better explains my goal.

Note (if it matters):The calculation being used to aggregate the data is

sum(iif([Account Type]="Income",[GLTxn Line Amount]*-1,[GLTxn Line Amount])).  If I just used GLTxn Line Amount (every GL transaction that hit a particular account) as my measure all of the income accounts would be negative - technically correct from the database's perspective but does not look right for reports. 

Please let me know my goal is possible with Tableau and if so what I need to do to get there.  If this is a common question I apologize - I tried searching for similar topics but was unable to find anything.  Thank you in advance.

Adam